Combustion in internal combustion engines produces vibratory forces which release sound waves. The sound waves are conducted through solids to adjacent structural parts and expand to the engine surface. The amplitude of the sound waves is dependent upon the mechanical impedances of the transmission path and upon the stimulated structural parts. The total movements of the sound waves are transformed according to the amplitude of radiation on the engine surface into sound transmitted by air and produce the audible and measurable engine noise.
Measures are known by which the oscillations of the structural parts surfaces can be reduced thereby reducing the radiation of sound transmitted by air. One particular solution has been proposed (MTZ 36 (1975) 5, p. 155) by which the surfaces of the most radiating structural parts are covered with muffling material.
The disadvantage of this solution is the very expensive application of muffling material, particularly onto structural parts of complicated form, as well as the vulnerability of nonmetallic muffling materials.
The present invention proposes an entirely different method for lowering the engine noise emissions.